Seismic surveying is a technique for generating information about an area below the earth's surface to identify subterranean structural features, such as voids or changes in composition. Petroleum companies, for example, use seismic surveying to explore for oil and gas reserves. Seismic surveying involves introducing one or more seismic waves into the area to be surveyed and sensing seismic activity at one or more locations on and/or below the surface of the earth in or near the surveyed area. The seismic waves may be generated, for example, by a controlled explosion or large “hammer” strike at the surface of the area to be imaged. One or more seismic sensors may be placed on and/or below the earth's surface to detect seismic activity caused by the waves. Such seismic activity typically includes seismic energy reflected back to the earth's surface as the seismic waves encounter structural discontinuities in the surveyed area. Seismic surveying systems use the seismic activity information collected by the sensors to generate information about subterranean composition and structure and may express the information in the form of paper traces or display images.
Seismic waves may be classified as compressional waves or shear waves. Compressional waves, sometimes referred to as primary waves, pressure waves or P-waves, are generally longitudinal in that the particles in the medium through which the waves travel vibrate along or parallel to the direction of travel of the wave energy. Shear waves, also referred to as secondary waves or S-waves, are generally transverse in that the particles in the medium through which the waves travel vibrate in a direction that is perpendicular or transverse to the direction of travel of the wave energy. Compressional waves and shear waves possess different characteristics. Compressional waves, for example, typically propagate at a higher speed than shear waves and are capable of propagating through fluid, while shear waves are not.
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